Blood flow measurement device

ABSTRACT

A blood flow velocity measurement device is devised where there exist such entities within the fluid which are detectable when axially flowing (passing by) an appropriate detector of known and well defined dimensions mounted onto a catheter. The entities produced for instance by a generator, when flowing by the detector, induced a known single response, the response bearing direct correlation to the flow velocity, in the form of direct reciprocity to the velocity and direct proportion to the length of the sensitive length of the detector. Autocorrelation of the function obtained by the overlap and pile-up of successive events is calculated and from the characteristic points in the autocorrelation function the axial velocity is inferred. The measurement is best when the probing beam is perpendicular to the flow.

FIELD OF INVENTION

The invention pertains to medical technology. It relates to fluid flowmeasurement in the field of medicine, and in particular to a blood flowmeasurement device.

BACKGROUND AND PRIOR ART

Fluid flow measurement plays an important part in modern medicine. Thisapplies both to pure diagnostic procedures as well as to direct controlof disease treatment. The various fluids which are, in this sense, ofinterest in the human body are very different in properties and includeblood, pus, urine, inhaled and exhaled air, drug infusions, etc. In allthe cases there exists a possibility of measurement of the flow usingthe Doppler effect provided there exist a wave reflecting ortransmitting entity within the said liquid. A basic property of theDoppler effect is that it takes place if there exists a velocity alongthe line connecting the wave transmitter and its receiver. For planewaves, the Doppler effect equals zero if the probing waves are at 90° tothe direction of movement of the said fluid. This means that only thevelocity component projected to the line connecting the wave transmitterand its receiver is measured. This limitation applies strictly only toplane waves, while for bundled waves (beams) one can theoreticallyobtain a low frequency Doppler effect due to the fact that the wavefronts in a beam are not plane (Censor D., W. L. Newhouse, T. Wonz, H.V. Ortega: Theory of Ultrasound Doppler Spectra Velocimetry forArbitrary Beam and Flow Configurations, IEEE Trans. on BiomedicalEngineering, 35 (1988)740). This, however, is a part of frequencyspectrum which is fully within the Doppler shift frequency spectrum ofother movements in the body, such as the movement of the blood vesselwalls, movement of the liver due to the heart beats, breathingmovements, etc. This makes the cited phenomenon hard (and oftenimpossible) to use, and in particular to use when the measurementprocess is unattended by interpreting humans (i.e. is used for someautomatic regulation). Cross correlation techniques for velocitymeasurement exist in the form of wave transmitter-receiver array systemswhere the correlation among different lines-of-sight is used to obtainthe velocity information. Due to the intrinsic physical impossibility toreduce the size of such a system the measurement is slowed down, thuscorrectly yielding only the low range of velocities encountered in thebody. In addition, to the said velocity limitation the said crosscorrelation method is limited to pulsed wave mode in the case whenvelocities axial to the measurement device are measured. (Kasai, C., K.Namekawa: Real-time two-dimensional blood flow imaging using anautocorrelation technique, Proc. IEEE Ultrasonic Symposium 1985, IEEE,p. 953). While this approach yields Doppler equivalent results when usedalong the scanner line-of-sight and can even measure slow perpendicularvelocities, it always requires full arrays of multiple transducers andis problematic in the measurement of the high velocities perpendicularto probing beams. In EP 228 070 assigned to Aloka Co.Ltd. the problem ofthe knowledge of the angle has been partially solved by using twoprobing beams under slightly different angles. The method requiresfairly elaborate electronic systems and is usable only when the acousticwindow is large enough to accommodate for the said two beams. In EP-144968 and JP-228 330 the time domain calculation is adjusted forsimplified calculation of the effects along the line-of-sight, yieldinga Doppler-like result. The time domain calculation method has furtherbeen described in EP-92841, JP-070479 and U.S. Pat. No. 4,573,477 withthe advantage of increasing and optimizing the sampling rate of theDoppler equivalent on-line measurement.

A larger number of prior inventors concern themselves with using thein-phase and quadrature detection components for further correlationcalculations with improved results in Doppler-like procedures. Thisapplies to EP-447597 which improves the measurable velocity by usingmultiple measurements and particular inter correlation calculations. InFR-2551213 the two quadrature demodulation components are used with twoauxiliary oscillators to obtain a Doppler equivalent result. InEP-266998 and U.S. Pat. No. 4,790,323 the quadrature detectioncomponents are on-line compared and yield a better turbulence estimationin Doppler measurements.

Yet other methods which use correlation calculations have been used fordifferent purposes and ways. For example in DE-3544477 and GB-2170972the autocorrelation calculation between the two Doppler shift componentsare used to reduce noise. Quadrature detectors and mixing oscillators inconjunction with autocorrelation calculation are used in EP-140726 tooptimize the number of samples and adjust them to flow velocity.

Calculations and measurements including transit time may be used toobtain the Doppler measured velocity profile as in EP-150672 where twobeams at different angles are used to ascertain the position of themeasurement volume and profile assessment. Other principles, such as aheating and cooling measurement system as in WO-9215239 are used forinstantaneous velocity and viscosity measurement (this is, however, afairly slow process).

Another possibility, to measure fluid velocity is the transit timemethod which does not rely on the scatter of ultrasound from red bloodcells, but on the fact that ultrasound propagates through moving mediumat a different speed compared to the speed in a still medium.Instruments based on this phenomenon have been described in a number ofpublished works on flow measurement devices ( e.g. potentiallyimplantable devices like one described by Franklin, D. L., Baker, D. W.,Rushmer, R. F.: Pulsed ultrasonic transit time flowmeter, IRE Trans.Bio-Med. Electron. 9: p.44, 1962; dual frequency devices, e.g. Noble F.W.: Dual Frequency Ultrasonic Fluid Flowmeter, The Rev. of ScientificInstruments, 39, no. 9, (1968), p. 1327 and intravascular, implantabledevices have been described using the same principle Plass, K. G.: A NewUltrasonic Flowmeter for Intravascular Applications, IEEE Trans. onBio-Med. Eng., BME-20, no. 1, (1973), p.154, and finally, particulartypes of interferometry, the phase-shift of upstream and downstreampropagating ultrasound has been described in Zarnstorff, W. C.,Castillo, C. A., Crumpton, C. W.: A Phase-Shift Ultrasonic Flowmeter,IRE Trans. on Bio-Med. Electronics, 9, (1962), pp 199-203). Thisapproach was abandoned when the sensitivity of ultrasound transducersand preamplifiers was improved enough in order to use the Dopplereffect. The main reason was apparently that the said transit timedevices always had to use two oppositely positioned transducers, makingthem less practical for implantation and usually useless for applicationfrom the body surface. Although this approach is now very rarely used,its potentials wait to be reevaluated with the improved materialstechnology. Some of such a development can be seen in U.S. Pat. No.4,227,407 to Drost who uses the previously described transit-timephenomena for interferometric blood flow measurement, with thedisadvantage of having to either exactly know the dimensions of theblood vessel or to have two implanted transducers.

U.S. Pat. No. 4,978,863 discloses an apparatus for determining flowrates of a fluid medium containing particles capable of backscatteringlight. The apparatus comprises a single probe means for illuminating asingle, finite and symmetrical sensor field of said medium with lightcapable of being backscattered by said particles, said probe meansincluding means for collecting backscattered light from said illuminatedsensor field, means for converting said collected backscattered lightinto voltage waveforms and autocorrelation means for measuring thebandwidth of said waveforms at discrete points in time to determinefluid medium flow rates. Each discrete point in time establishes a timedelay corresponding to 50% the decorrelation decay between initial andfinal values. Each time delay corresponding to 50% decorrelation isconsidered to be inversely proportional to the approximately averageflow rate in the asymmetrically illuminated part of the stream.

EP-A-0 474 957 describes a blood flow measurement device according tothe preamble of claim 1, wherein at least one Doppler measurementultrasonic piezoelectric transducer means is arranged and mounted onto acatheter at the circumference thereof in a manner as to be able ofgenerating an essential narrow directivity characteristic adjacent tothe catheter. In one embodiment two transducer means are arranged,spaced apart from each other whereby the directivity characteristicscross each other to form a sensitivity volume.

It is an object of the present invention to provide an improved bloodflow measurement device particularly suitable for cases when the anglebetween the flow and the wave beam or other sensing directivityfunctions used for the measurement approaches 90° or is exactly 90° andwhen the velocity direction is known or immaterial but the measurementof high velocities is important.

This object is attained by the blood flow measurement device accordingto claim 1. Preferred embodiments are described in the dependent claims.

SUMMARY OF THE INVENTION

A blood flow velocity measurement device is devised where there existsuch entities within the fluid which are detectable when axially flowing(passing) by an appropriate detector of known and well defineddimensions. The said entities, when flowing by the said detector, inducea known single response, the response bearing direct correlation to theflow velocity, in the form of direct reciprocity to the said velocityand direct proportion to the length of the sensitive length of the saiddetector. Autocorrelation of the function obtained by the overlap andpile-up of successive events is calculated and from the characteristicpoints in the said autocorrelation function the axial velocity isinferred. The measurement is best when the probing beam is perpendicularto the flow.

An undirectional (single-side-band) device for measuring fluid flowvelocity at 90° has been invented. The device applies to any fluid whichcontains elements (further called scatterers) which can be detected asdistinctive entities by some method with well defined directivitycharacteristics. The device consists of a detector (or detector set)capable of detecting the said entities as they enter the sensitivityvolume (within the sensitivity characteristic) as well as the momentwhen they leave the sensitivity volume (further called beam). Thevelocity of the fluid which passes through the sensitive volume(crossing it by its width) is the ratio of the sensitivity volume widthand the time spent within it. For the normal case when the number ofscatterers is so large that their crossing times overlap, the resultingsignal is a combination of overlapped single crossing characteristics.The information about the crossing velocity is extracted by on-linecalculation of the autocorrelation function of the described function ofthe overlapped crossing signals of a multitude of scatterers. Theinformation of the single crossing time across the beam is, depending onthe signal processing type, the delay time at the autocorrelationfunction zero crossing or its negative minimum (for positive crossingsignals). The type of scatterers and the type of beam are basicallyirrelevant, but include pulsed wave and continuous wave ultrasound withultrasound scatterers, light waves and light scatterers, detectors ofion clusters and ion clusters in liquid, magnetism detectors (inductiveor otherwise) with ferromagnetic particles dispersed in the liquid, etc.

SHORT DESCRIPTION OF FIGURES

FIG. 1 represents a general situation where a sensing beam istransmitted into flowing fluid at a general angle and where the sametransmitter is used for reception of the reflected waves.

FIG. 2 represents a block diagram of a device capable of using the datafrom the waves incident from flowing fluid for flow measurement as perFIG. 1.

FIG. 3 represents the information flow diagram implemented when all or apart of the functions are performed with a computer (digital oranalogue).

FIG. 4 represents a perspective illustration of an ultrasonictransmitter receiver transducer mounted onto an intraluminal catheter.

FIG. 5 represents an illustration of two piezoelectric transducersmounted onto a catheter.

FIG. 6 represents an illustration of paired wave (piezoelectric orlight) transducers mounted onto a catheter.

FIG. 7 represents a perspective illustration of a general generatingdevice means mounted on a catheter means capable of generation and/orinjection into flowing fluid of entities.

FIG. 8 represents a three projection illustration of the deviceillustrated in FIG. 7 with connection leads means.

FIG. 9 is an illustration of one embodiment of a device capable ofdetecting entities with magnetic polarization properties.

FIG. 10 represents an illustration of a light transmitting-receivingmatrix device.

FIG. 11 shows the heart having implanted a lead comprising the aorticflow measurement assembly.

FIG. 12 shows the caudal view on the heart illustrating the aortic flowmeasurement.

DETAILED DESCRIPTION OF THE EMBODIMENTS

Referring to FIG. 1 we can describe the basic principle of the velocitymeasurement device as follows. The beam 1 shown here is in fact asensing directivity characteristic of a general probing device 3 whichmay be attained with or without the use of any sort of waves. Adetectable entity (e.g. a scatterer of ultrasound waves) passes the areaoccupied by the sensitivity characteristic 1, e.g. an ultrasound beam,at the point where the width of the beam equals d. The scatterer passesat a general angle φ. The velocity V of the scatterer can be dividedinto components V_(h) and V_(v). The velocity component V_(h) equals tothe width d divided by the time the scatterer spends within the width d.The time spent within the beam can be measured by electronic circuitswhich detect the presence or the absence of the scatterer within thebeam. The velocity component V_(v) can not be measured in this way. Letit be noted that this measurement method yields a means of measuring thevelocity component perpendicular to what is possible with the Dopplermethod, i.e. the Doppler effect for plane waves equals zero if thevelocity is purely V_(h). Unless the scatterer velocity is exactly alongthe beam axis, the total velocity can be calculated from the, thusmeasured, velocity V_(h) by division with the sine of the angle φ. It isimportant to note that in case of one single beam the direction of the,thus measured velocity V_(h) can not be inferred from this measurement.On the other hand this method yields the absolute value of the saidvelocity component in the physically fastest possible way.

Now, if there are more than one scatterers passing through the beam, andif their appearance is dense enough, the signals which signal theirpresence within the beam may overlap. In fact, if there is a largemultitude of such scatterers, the individual features of the signal asindividual scatterers enter and leave the beam will apparently be lostwithin the pile-up variable signal.

However, the autocorrelation function of such a pile-up process must bymathematical laws contain a characteristic point, a discontinuity orextreme at the autocorrelation delay time equal to the time needed forindividual scatterers to pass across the beam width d. Theautocorrelation function is defined as the stochastic expectation of thevalues of a random process that are the delay time τ apart and, whichare multiplied by each other. (A. Papoulis: Probability, RandomVariables and Stochastic Processes: McGraw Hill Inc., 1965., page 359).

Therefore, referring to FIG. 2, the signals detected from the flow of ascattering medium are fed to an autocorrelation function calculator(digital or analogue). The time period until the occurrence of thecharacteristic point, normally the first minimum of the autocorrelationfunction is taken to be of the value equal to the time needed by thesaid scatterer to cross the distance d. Velocity is calculated bydivision of the width d of the said beam 1 with the thus measured timeof occurrence of the characteristic point.

These and the following calculations and evaluations may be carried outin a suitable information processing device and be implemented byhard--and/or software, or by hardware alone.

Referring to FIG. 3, a flow diagram for the calculation is devised asfollows: The signal reflected from the flow area of interest in the flow2 is input in real time. The signal is preprocessed, e.g.differentiated, frequency filtered, etc. Autocorrelation orautocovariance are calculated on the inflowing data. A timer circuitinduces evaluation of thus gathered autocorrelation function and thesearch for the characteristic point, i.e. the first discontinuity whichoccurs at the point where the delay τ is equal to the transition time ofa single scatterer across the beam 1 of FIG. 1. The beam width d isdivided by the resultant time which yields the required velocitycomponent V_(h). For single scatterer signal starting with a positiverise and ending with a decrease to initial level, the saidcharacteristic delay appears at the delay point where the negative slopeof the autocorrelation function changes the slope from the initialnegative slope to zero or positive slope. Thus the characteristic delaycan be detected by detecting this change in the slope of theautocorrelation function.

Referring to FIG. 4 showing the preferred embodiment of the invention(best mode), a device 101 generating a sensitivity area 102 is mountedonto a catheter means 11, the said sensitivity area having acharacteristic axial (for the catheter) dimension d. The sensitivitycharacteristic is for simplicity shown as square, although it can haveany physical form characteristic of wave beams created by directionalantennas. The catheter means 11 can have a lumen 12 to accommodate fordifferent functions including that of bringing to and taking away ofsignals for the device 101. The device 101 comprises an ultrasonictransmitter receiver transducer which can be used for echo detection ofparticles flowing parallel to the catheter axis and crossing thesensitivity area 102. The same said device serves both as thetransmitter and as the receiver of the waves. Width d is thecharacteristic dimension which appears in the velocity calculations.

FIG. 5 represents an illustration of two piezoelectric transducersmounted with one of their dimensions, preferably with their longestdimensions parallel onto a catheter where one of them is used ascontinuous wave transmitter and the other as a continuous wave receiverof waves scattered from the axially flowing fluid from within thesensitive area 18 which is the cross section of the transmissioncharacteristic 94 and reception characteristic 95 obtained by tiltingthe beams from the said transducers by the use of tilting devices(lenses) 97 and 98.

Referring to FIG. 5, we can define a sensitive area in the sense of theprevious text by application of two cylindric or otherwise formedpiezoelectric transducers 91 and 92 which have glued on or otherwisefixed ultrasound beam tilting lenses 98 and 97 respectively which tiltthe directivity characteristics 94 and 95 respectively, of the twodevices in such a way as to overlap within an area 18. This area 18 hasthe properties of the generally outlined area 102 from FIG. 4 or thegeneralized beam of FIG. 1. The sensitivity area can be obtained bycontinuous transmission of waves from one of the said transducers (e.g.91) and reception of the scattered waves by the other of the transducers(e.g. 92). The transmission and reception can be continuous orsynchronized pulsed transmission and reception. The electrical signalsneeded to actuate the said transducers are fed to them and taken fromthem to appropriate electronic circuits by conductors built into thecatheter means 11 body (not shown).

FIG. 6 represents an illustration of paired wave (piezoelectric orlight) transducers (14 and 15 in case A and of a different form 16 and17 in case B) mounted onto a catheter, arranged differently from thosefrom FIG. 5, where one of them is used as continuous wave transmitterand the other as a continuous wave receiver of waves scattered from theaxially flowing fluid. The directivity characteristics in case A aredesignated by 25 and 24 and in case B by 26 and 27, while theirintersections are designated by (18) and (19) respectively. The two sets(embodiments) of transmitters and receivers of waves (light orultrasound) are positioned in such a way as to enable one of the devicesto transmit the said waves into the scattering medium and the otherdevice to receive the scattered waves from the area where the twodirectivity functions overlap.

Referring to FIG. 6 more in detail, we see an illustration of a cathetermeans 11 with a lumen 12 with two embodiments of lighttransmitter-receiver sets, A and B. In embodiment A the transmitter andthe receiver of light waves 14 and 15 respectively are preferablyfocussed and their directivity characteristics 24 and 25 respectivelyoverlap at an area 18. In the embodiment B the wave transmitter 16 andthe receiver 17 have been shown of a square form with directivitycharacteristics 26 and 27 overlapping within an area 19 of anessentially square form. The essentially square form of the sensitivityarea has the property of having equal transit lengths for all thescatterers flowing within the fluid around the said catheter means inaxial direction relative to the said catheter means. The lighttransmitter and receiver have the property of being able to transmit andreceive the light waves.

Referring to FIG. 7, there is an illustration of the said catheter 11having mounted thereon a general property generator 34 capable ofgeneration and/or injection into flowing fluid of entities (e.g. ions ormagnetic dipoles) detectable within sensitivity characteristics 45 ofreception devices means 35 and connected to outside circuits via leadsput into the lumen 12 of the catheter means. This ig. represents anillustration of an apparatus in which one device generates a detectableproperty in the liquid, (e.g. ionizes it), and the other detector meansmeasures the amount of the axially passing detectable entities (e.g. ionclusters) using the method of extracting the velocity data outlined inFIG. 1. The generator and the two general property detectors 35 haverespective directivity functions 44 and 45. The property (e.g.ionization, magnetization or the like) is imposed onto the particlesflowing in the liquid at velocity V and detected within the sensitivityarea 45 of the property detectors. The device principle outlined in thedescription of FIG. 1 to 3 is then applied to the signal thus obtained.

Referring to FIG. 8, which is a three projection illustration of thedevice from FIG. 7, illustrating that the said generation means 34 andthe said reception means 35, respectively shall be connected to theproximal side of the said catheter 11 using general conductors 51, 52,53 led through the lumen 12 and connected or connectable to electroniccircuits performing the operations outlined in FIG. 2 and 3.

FIG. 9 is an illustration of one embodiment of a device capable ofdetecting entities with magnetic polarization properties flowing axiallywith regard to the catheter means 11 by virtue of orienting the saidmagnetic dipoles between the south S and north N pole of a magnet with agap 75 between them and of a length L and by detecting the change inmagnetization of the dipoles by sensing coils 71 and 72.

In this embodiment of a magnetic detector of magnetizable entitiesflowing axially with the liquid parallel to dimension L in FIG. 9 suchmagnetizable entities (e.g. particles) abruptly change their magneticorientation when entering the magnetic field between the south S andnorth N pole of a magnet where the distance among the said pole piecesis known and defined 75. The sudden change of the magnetic polarizationand depolarization at the entrance and at the exit can be detected bysaid detection coils 71 and 72 and fed to signal processing systemsaccording to FIG. 2 and 3 in order to calculate the axial velocity ofthe said flowing particles.

FIG. 10 represents an illustration of a light transmitting-receivingmatrix device where on a support plate 201 light transmitters 202 andlight receivers 203 are arranged in a dense way such as to yield ahomogeneous light field in front of the device and the scattering of thelight waves by scatterers flowing in front of the device are detectedcontinuously by receivers 202 which jointly act as one receiver of thelength L and width W. Distances 205 and 206 can be made as small asnecessary.

Referring to FIG. 10 more in detail, the composite lighttransmitter-receiver device as seen from the front, comprises lighttransmitting devices 202, e.g. LEDs and light reception devices 203,e.g. photo transistors packed densely enough (with distances 205 and 206small enough) to act as a joint transmitter-receiver device detectinglight scatterers as they pass in front of the device. The devices 202and 203 are preferably defocussed in such a way as to create a quasicontinuous field of light in front of the whole device. If the flow isparallel to the dimension L then this is the characteristic dimension inthe sense of calculation outlined in FIG. 2 and 3, and the same relatesto dimension W if the flow is parallel to it.

FIG. 11 and 12 disclose the principle of aortic flow measurement bymeans of the transducer assembly disclosed in previous Fig. and relevantdescription. The aortic flow measurement can be utilized for cardiacelectrotherapy control. Disclosed measurement system is feasible to beincorporated within an implantable electrotherapy device.

As it is known in the art, every ventricular contraction produces theaortic flow wave. Accordingly, the stroke volume can be calculated fromthe waveform of the aortic flow, assuming that the aortic cross-sectionarea is a known previously measured parameter. Therefore this device canbe used for rate variation in rate responsive pacing, pacing capturemonitoring, tachycardia detection as well as differentiation,ventricular fibrillation detection, and left ventricular systolicmyocardial function estimation. These physiological principles weredisclosed in numerous prior art of cardiology and echocrdiography.

FIG. 11 shows the heart opened at the right atrial appendage 161. Thereare tricuspid valve 162, fossa ovalis 163, coronary sinus valve 164 andcrista terminalis 165 within the right atrium. The vena cava superior166 and the vena cava inferior 167 as well as the pulmonary artery 168and the aorta 169 with truncus pulmonalis 170 are disclosed. The leftatrium 171 with right superior pulmonary vein 172 as well as with rightinferior pulmonary vein 173 are shown. The right ventricular apex 174 isdisclosed as well as the residue of the pericardium 175. The pacemakerlead 176 is implanted through the vena cava superior 166 and rightatrial cavity through the tricuspid valve 162 in the right ventriclewith its tip (not shown) in the area of apex 174. The lead 176 comprisesan ultrasonic transducer assembly 177 which produces the measurementultrasonic field 178 directed towards the aortic arch 179.

FIG. 12 shows the caudal view on the heart having the analogousdesignations for same elements which are shown on previous FIG. 11. Thelead 176 is implanted through the superior vena cava 166 as disclosed onthis axial view. As it is clearly demonstrated in this projection, theultrasonic measurement field 178 produced by transducer assembly 177 isdirected towards the aortic arch 179. Ideal situation is disclosed,whereby ultrasonic beam is perpendicular towards the aortic flow.

We claim as our invention:
 1. A blood flow measurement device formeasuring a flow perpendicular to a probe beam, comprising:a catheterinsertable into a blood vessel in the body of a subject, said catheterhaving a distal end and a proximal end and a length extendingtherebetween; at least one detecting device means mounted onto saidcatheter at a position along said length without impeding insertion ofthe catheter into the blood vessel, for emitting a probe beam having adirectivity characteristic which defines a sensitivity volume fordetecting entities flowing through said sensitivity volume and forgenerating detection signals corresponding to detection of saidentities; electronic circuitry means for measuring and performingcalculations on the signals generated by said detecting device means toobtain calculation data signals; and evaluation means responsive to saidcalculation data signals for determining a velocity of flow of saidentities therefrom, said electronic circuitry means comprising means forperforming time autocorrelation calculations and said evaluation meanscomprising means for detecting a signal characteristic in saidcalculation data signals for obtaining a characteristic delay timewithin an autocorrelation function and for dividing the sensitivityvolume in a direction of said flow with said characteristic delay timefor identifying a velocity component of the blood flow in saiddirection.
 2. A blood flow measurement device as claimed in claim 1wherein said detecting device means comprises an ultrasonic transducerand means for operating said ultrasonic transducer in at least one of apulse-echo mode and a continuous wave mode.
 3. A blood flow measurementdevice as claimed in claim 2 wherein said ultrasonic transducer includesa plurality of piezoelectric transducer elements of differentgeometrical forms which, in combination, produce a defined directivitycharacteristic, said piezoelectric transducer elements being mountedonto said catheter.
 4. A blood flow measurement device as claimed inclaim 3 wherein said catheter has a longitudinal axis along said lengththereof and wherein said piezoelectric transducer elements produce adirectivity characteristic having an axis disposed substantiallyperpendicular to said longitudinal axis.
 5. A blood flow measurementdevice as claimed in claim 4 wherein said means for operating saidultrasound transducer comprise means for operating said ultrasoundtransducer in said pulse-echo mode with a sensitivity for detectingultrasound scattered from erythrocytes as said entities in said flow. 6.A blood flow measurement device as claimed in claim 3 wherein saidplurality of piezoelectric transducer elements includes at least onepair of piezoelectric transducer elements, each piezoelectric transducerelement in said at least one pair having a directivity characteristicand the respective directivity characteristics of said piezoelectrictransducer elements in said at least one pair overlapping and defining avolume outside of said catheter comprising said sensitivity volume, andsaid piezoelectric transducer elements in said at least one pair eachhaving a longest dimension which extends parallel to said length of saidcatheter.
 7. A blood flow measurement device as claimed in claim 1wherein said detecting device means comprise at least one pair ofoptical transducer elements, each optical transducer element in saidpair having respective transmission and reception directivitycharacteristics, and the respective transmission and receptiondirectivity characteristics of said optical transducers in said at leastone pair overlapping and defining a volume outside of said cathetercomprising said sensitivity volume.
 8. A blood flow measurement deviceas claimed in claim 1 wherein said detecting device means comprise iongenerator means for generating ions in blood in said blood vessel, andion detector means, having said sensitivity volume disposed in frontthereof, for detecting a density of ions in said sensitivity volume. 9.A blood flow measurement device as claimed in claim 1 wherein saiddetector device means comprise magnetic polarization detector means,having said sensitivity volume in front thereof, for detectingmagnetically polarized entities in said sensitivity volume.
 10. A bloodflow measurement device as claimed in claim 9 wherein said magneticpolarization detector means comprise a ferromagnetic particle detectiondevice having a length defining said sensitivity volume, saidferromagnetic particle detection device detecting a presence or absenceof at least one ferromagnetic particle in said sensitivity volume.
 11. Ablood flow measurement device as claimed in claim 10 wherein saidferromagnetic particle detection device comprises a north-southpolarized pole piece pair which generate a magnetic field in saidsensitivity volume, and a pair of detection windings which detectchanges in said magnetic field as a result of said at least oneferromagnetic particle entering and leaving said sensitivity volume. 12.A blood flow measurement device as claimed in claim 11 wherein saiddetecting device means comprise a light transmission-detection devicedisposed on a surface of said catheter and containing a plurality ofdensely packed light sources and detectors, each light source anddetector having a directivity characteristic and the respectivedirectivity characteristics of said light sources and detectorsoverlapping at a defined distance above said surface, and each lightsource and detector having a substantially inverse square sensitivitycharacteristic with respect to distance.
 13. A blood flow measurementdevice as claimed in claim 11 wherein said catheter comprises electricalconductors running along said catheter from said detecting device meansto the proximal end of said catheter.
 14. A blood flow measurementdevice as claimed in claim 13 wherein said catheter comprises electricalconnectors respectively terminating said conductors at said proximal endof said catheter.
 15. A blood flow measurement device as claimed inclaim 14 wherein said electronic circuitry means is disposed at saidproximal end of said catheter, connected to said electrical connectors,and wherein said electronic circuitry means includes means foractivating said at least one detecting device means and for detectingelectrical signals from the activated detector device means.
 16. A bloodflow measurement device as claimed in claim 15 wherein said evaluationmeans are disposed at said proximal end of said catheter, connected tosaid electronic circuitry means.
 17. A blood flow measurement device asclaimed in claim 1 further comprising display means connected to saidevaluation means for displaying a representation of said velocitycomponent.
 18. A blood flow measurement device as claimed in claim 1further comprising transmission means, connected to said evaluationmeans, for transmitting a representation of said velocity component to alocation remote from said evaluation means.